1. Field of the Invention
The present invention relates to microtip flat panel displays. It more particularly applies to the realization of a cathodoluminescent anode for such flat panel displays and, in particular, to anode luminescent element connections for operation with a switched anode.
2. Discussion of the Related Art
FIGS. 1 and 2 are a cross-sectional view and a perspective view, respectively, of the structure of a microtip flat panel display according to the present invention.
Such a microtip flat panel display is mainly constituted by a cathode 11 including microtips 10 and by a gate 5 that is provided with holes facing the microtips 10. Cathode 11 faces a cathodoluminescent anode 12 having a glass substrate 2 that constitutes the screen surface.
The operation and the detailed structure of such a microtip flat panel display are described in U.S. Pat. No. 4,940,916 in the name of Commissariat a l'Energie Atomique.
Cathode 11 is constituted, on a glass substrate 7, by cathode conductors 3 disposed in columns. The cathode conductors 3 are generally coated with a resistive layer (not shown) for the homogeneity of the electron emission. An insulating layer 4 to insulate the cathode conductors 3 from gate 5. Holes are respectively provided in the gate layer 5 and the insulating layer 4 to accommodate the microtips 10 that are formed on the resistive layer. Gate 5 is arranged in rows L1, L2, L3. The intersection of a gate row L and of a cathode column 3 defines a pixel. For the sake of simplification, only a few microtips 10 are represented in FIG. 2 at the intersection of a row L with a column 3. In practice, there are several thousands of microtips 10 per pixel.
This device uses the electric field generated between cathode 11 and gate 5 to extract electrons from microtips 10 toward phosphor elements 8 of anode 12 through a vacuum 6.
In the case of a color display, anode 12 is provided with alternate strips of phosphor elements 8, each of which corresponding to a color (blue, red, green). Each strip is electrically insulated from its two adjacent strips. Phosphor elements 8 are deposited onto electrodes 7 that are constituted by corresponding strips of a transparent conductive layer, such as indium-tin oxide (ITO). The strips are disposed in parallel with the cathode columns 3. A group of three strips (one for each color) facing one cathode column. Thus, the width of a group of strips of anode 12 corresponds to a pixel width. The group of blue, red, and green strips are selectively polarized with respect to cathode 11 so that the electrons extracted from the microtips 10 of one pixel are selectively directed toward the phosphor elements 8 facing each of the colors.
Conventionally, all the strips of the same color are electrically interconnected, outside the useful surface of the display, to an electronic control system (not shown). On the cathode side, each cathode column 3 and each gate row L is individually connected to the electronic control system.
An image is displayed during a frame period (for example 20 ms) by adequately polarizing anode 12, cathode 11 and gate 5 through an electronic control system. During a frame period, the group of strips of phosphor elements 8 of a same color is sequentially polarized, that is, each group is polarized for a sub-frame period corresponding to one third of the frame period (for example 6.6 ms). Display is performed line after line, by sequentially polarizing the gate rows for a "line time" during which each cathode column is raised to a potential that depends upon the brightness of the pixel to be displayed along the current row in the selected color. The polarization of columns 3 of cathode changes at each new row of the line scan. A "line time" (for example 13.7 .mu.s) corresponds to the duration of one sub-frame divided by the number of gate rows.
Therefore, the groups of strips of phosphor elements 8 are sequentially raised to a potential that permits to attract the electrons emitted by the microtips 10. This potential depends upon the distance (vacuum 6) which separates the cathode/gate from the anode and is, for example, higher than 250 V. The gate rows are sequentially polarized during a sub-frame period. A specific row L is raised to a potential (for example 80 V) whereas the remaining rows are at a zero potential during the "line time" of the current row. The cathode columns, whose potential represents at each line the brightness of the pixel defined by the intersection of column 3 with row L in the selected color, are raised to respective potentials between a maximum emission potential and the absence of emission of potential (for example, 0 and 30 V, respectively). The values of the polarization potentials are selected as a function of the phosphor elements 8 and microtips 10. Conventionally, below a potential difference of 50 V between the cathode and the gate, there is no electron emission and the maximum emission corresponds to a potential difference of 80 V.
A drawback of conventional flat panel displays lies in that electrons emitted by microtips 10 of a specific column 3 of cathode 11 tend to excite the strips of phosphor elements 8 of the same colors facing two adjacent columns 3. Indeed, although two strips of a same color are separated by two strips of a different color, the distance (approximately 0.2 mm) between the phosphor elements 8 and microtips 10 leads the electrons to deviate towards the nearest strips of the same color. This illumination of adjacent pixels is increased when groups of strips of phosphor elements 8 are misaligned with respect to the cathode columns, which may occur during assembling of the display.
In the case of a monochrome display, the simplest way to realize an anode 12 includes depositing, over the whole substrate 2 of anode 12, a conductor 7 coated with continuous phosphor elements 8. Anode 12 is permanently polarized. The selection of the excited areas of the display by the electrons emitted by microtips 10 is controlled by the respective polarizations of cathode columns of gate rows. The drawbacks of the color displays are still more accentuated in such monochrome displays.